1. Field of the Invention
The invention generally relates to current sensing circuits and methods; and in particular, the present invention relates to a ratiometric current sensing circuit for accurately sensing the current flowing through a power-controlling pass device.
2. Background of the Invention
In circuits employing a power switch for power switching or power distribution functions, there is often a need to sense the current passing through the power switch. For example, current sensing is needed to monitor the load current passing through the power switch and the load coupled to the power switch. Current sensing is also needed to control and limit the load current in order to prevent damage to the load or to the power switch itself. Power switches are commonly implemented as n-channel or p-channel MOS devices. Although the current through the power switch can be sensed directly by placing a resistor in series with the power switch, this arrangement is undesirable because the resistor conducts the entire current through the power switch; resulting in a large power dissipation. Instead, a ratiometric current sensing technique is typically used for MOS power switches. In ratiometric current sensing, the current through the power switch is measured using a sense device which matches the power switch in electrical characteristics but is smaller by a known factor. The current through the sense device, which is a known ratio of the current through the power switch, is measured using a resistor connected in series with the sense device. The size of the sense device can be made small enough such that the current through the sense device is measured without undesirable power dissipation.
A conventional ratiometric current sensing circuit for use with a MOS power switch is illustrated in FIG. 1. Current sensing circuit 10 for sensing the current through a power device MPower and a load 13 includes a sense device MSense and a resistor RSense connected in series. Power device MPower and sense device MSense are matching n-channel MOS transistors. Sense device MSense is chosen to be K times smaller than power device MPower. Typically, K is in the range of 1000 or more. The gate terminals of power device MPower and sense device MSense are connected together and the source terminals of both devices are connected together to a ground terminal (node 15). Therefore, power device MPower and sense device MSense are driven with identical gate to source voltages. An input voltage Vin from an input voltage source 12 is applied across load 13 and power device MPower. A load current flowing through load 13 is equivalent to the drain current IDS,P of power device MPower.
Resistor RSense is connected between the drain terminal (node 14) of power device MPower and the drain terminal (node 16) of sense device MSense and is used to measure the current flowing through the sense device MSense. As long as the voltage across resistor RSense is small compared to the drain-to-source voltage of MSense, the drain-to-source voltages across power device MPower and sense device MSense are essentially equal. Since the power device and the sense device have the same drain-to-source voltages and the same gate-to-source voltages, the drain current IDS,S of sense device MSense is essentially IDS,P/K. A voltage drop develops across resistor RSense which is equal to the product of the drain current IDS,S of sense device MSense and the resistance of resistor RSense.
The sensed current of sense device MSense and the sensed voltage of sense resistor RSense can be used to control circuit protection mechanisms for preventing excessive current flow in power device MPower and load 13. To that end, current sense circuit 10 further includes an error amplifier 20, a reference current source 19, and a reference resistor RRef. Reference current source 19 provides a fixed reference current IRef0 which flows through reference resistor RRef and generates a reference voltage across the reference resistor. Reference resistor RRef and sense resistor RSense are either matching resistors having the same resistance values or resistors having ratioed resistance values. Error amplifier 20 compares the voltage across reference resistor RRef (node 18) and the voltage across sense resistor RSense (node 16) and provides a control signal on lead 17 to the gate terminals of sense device MSense and power device MPower. In operation, the reference current IRef0 is selected so as to set the current limit of power device MPower. Error amplifier 20 operates to limit the power device""s current whenever the sensed voltage at sense resistor RSense is equal to or exceeds the reference voltage generated by reference resistor RRef. When a current limit condition is detected, error amplifier 20 regulates the gate-to-source voltages of power device MPower and sense device MSense to limit the current through the sense device to the maximum allowable current value of IRef0.
As mentioned above, in current sense circuit 10 of FIG. 1, as long as the voltage drop across sense resistor RSense is negligible as compared to the voltage drop across sense device MSense, the drain-to-source voltages across the power device MPower and the sense device MSense are essentially equal and the current through the sense device tracks the current through the power device. The drain current IDS,P through power device MPower and load 13 is given by:                                           I                          DS              ,              P                                ⁢                      xe2x80x83                    ≤                      K            *                          I                              DS                ,                S                                      *                                          R                Ref                            /                              R                Sense                                                    ,                                          xe2x80x83                ⁢                  =                      K            *                          I              Ref0                        *                                          R                Ref                            /                                                R                  Sense                                .                                                        
Through the use of a scaled-down sense device, current sensing circuit 10 operates at a low power dissipation level because the sensed current IDS,S is only a fraction of the power device""s actual current. Furthermore, current sensing circuit 10 is applicable when the power device is biased either in the saturation region or in the linear (triode) region.
However, conventional current sensing circuit 10 has a significant drawback. In particular, conventional current sensing circuit 10 becomes grossly inaccurate when the power device is operated in the linear region where the drain-to-source voltage across the power device is small. In this case, the voltage drop across the sense resistor is no longer negligible and the drain voltage at the sense device does not track that of the power device. Thus, sense device MSense grossly underestimates the power device""s current.
For sense device MSense to measure the power device current accurately, the terminal conditions of the two devices should be equal. That is, the gate-to-source voltages and the drain-to-source voltages should be the same for both devices. However, by virtue of the use of sense resistor RSense; some voltage is dropped across the sense resistor. Consequently, the drain voltage at sense device MSense is less than the drain voltage at power device MPower. In the case where the drain-to-source voltage across the power device is large, the voltage drop across the sense resistor is negligible and the drain-to-source voltages of the power and sense devices are essentially equal. However, when the drain-to-source voltage across power device MPower is small, the voltage drop across resistor RSense is large compared with the drain-to-source voltage of power device MPower such that the drain voltage of the sense device is significantly less than the drain voltage of the power device. The disparity in the drain voltages results in a disparity in the drain current of the two devices such that the sense device grossly underestimates the current flow in the power device.
FIGS. 10a-c are graphs of the current and voltage characteristics obtained by simulation of the conventional current sensing circuit 20 in FIG. 13. Current sensing circuit 20 is constructed in the same manner as conventional current sensing circuit 10 with the only exception that the load, including load resistor Rload having a resistance value of 2 ohms and load voltage source vLoad, is coupled to the source terminal of the power device Mout. FIGS. 10a-c illustrate the characteristics of current sensing circuit 20 in response to a linearly ramped load current and to a short-circuit condition at the load. In FIGS. 10a-c, current sense circuit 20 is operated at an input voltage Vin of 3.3 volts. Curve 178 of FIG. 10a illustrates the behavior of the load current through load resistor Rload. Curve 174 of FIG. 10b illustrates the gate voltage VGate as applied to both the sense device and the power device. Curves 170 and 172 of FIG. 10c illustrate the voltage at reference resistor RRef (VRef) and the voltage at sense resistor RSense (VSense), respectively, with reference to the input voltage Vin. That is, curve 170 is actually Vin-VRef and Curve 172 is Vin-VSense. Here, reference current source iRef sets the current limit of power device Mout to be 250 mA and sets the reference voltage VRef to 50 mV.
From a time zero to a time 0.75 ms, the load current increases linearly. The gate voltage (curve 174 of FIG. 10b) increases to a maximum value of 8 volts to allow the power device Mout to carry the necessary load current. Meanwhile, the sensed voltage VSense slowly increases until the sense voltage VSense reaches the reference voltage VRef (50 mV) at a time of 0.5 ms, indicating that the current limit condition is reached. Current sense circuit 20 limits the load current to a value of approximately 609 mA (curve portion 178a of FIG. 10a), instead of the intended 250 mA current limit. The excessive current limit value under the ramped current condition is caused by sensing inaccuracy when the power device is biased in the linear region. For instance, at about 0.5 ms, the load current is slowly ramped up to about 600 mA. The voltage Vout at the source terminal of power device Mout is the voltage across load resistor Rload and the load voltage source vLoad which is equal to 1.2 volts plus 2.0 volts. Thus, voltage Vout is 3.2 volts. The drain-to-source voltage VDS across power device Mout is only 100 mV (3.3 volts of Vin minus 3.2 volts of Vout) and power device Mout is biased in the linear region. In this regime, the 50 mV voltage drop across sense resistor RSense (denoted R1 in FIG. 13) is significant in comparison with the VDS of the power device (100 mV). The drain-to-source voltage of sense device MSense is reduced to only 50 mV and does not approximate the drain-to-source voltage of the power device. The drain-to-source voltage disparity causes sense device MSence to grossly underestimate the power device""s current and current sensing circuit 20 does not limit the load current until the load current reaches 609 mA, far exceeding the 250 mA intended current limit.
However, when a short circuit load is applied (at time 0.75 ms), almost the entire input voltage Vin of 3.3 volts is applied across power device Mout and sense device MSense and both devices are in saturation. Specifically, voltage Vout is only the voltage drop across the load resistor which is 0.52 volts (260 mA*2 xcexa9). Thus, the drain-to-source voltage across power device Mout is 2.78 volts. The sensed voltage VSense, being 50 mV (curve 172), is only a small fraction (1.7%) of the drain-to-source voltage of the power device. Therefore, under the short-circuit load condition, the disparity between the drain-to-source voltages of the power device and the sense device is small and sense device MSense can accurately sense the power device""s current. Current sense circuit 20 thus limits the current of the power device by lowering the gate voltage (curve 174) to about 1.5 volts. The load current is regulated down to 260 mA (curve portion 178b of FIG. 10a), closely approximating the intended 250 mA current limit. As can be observed in FIG. 10a, the value of the current limit under the ramped current condition is significantly higher than and the current limit under the short-circuit condition. The great disparity in the current limit values (a 135% discrepancy) is an indication of the sensing inaccuracy of the conventional current sensing circuit when the power device is biased in the linear region.
One prior art technique to improve the accuracy of the convention current sensing circuit is illustrated in FIG. 2. In current sensing circuit 30, a bipolar comparator, made up of pnp bipolar transistors 41 and 42, is used to keep the voltage drop across the sense resistor RSense small. However, current sensing circuit 30 is only able to limit the voltage drop across RSense to about 10 mV and the result is still unsatisfactory since the values of the current limits between a ramped load current and a short circuit condition still vary by over 60 percent.
Therefore, it is desirable to provide a ratiometric current sensing circuit which can accurately sense the current through a power device for all values of drain-to-source voltages at the power device. In particular, it is desirable to provide a ratiometric current sensing circuit which can sense the current through a power device accurately even when the power device is biased in the linear region.
A circuit for sensing a first current flowing through a load and a power-controlling pass device is described. In one embodiment, the load and the pass device are connected in series between a first supply voltage and a second supply voltage. The circuit includes a sense device coupled between a first node and the second supply voltage and a sense resistor coupled between the first node and a second node between the load and the pass device. The sense device has a smaller dimension than the pass device. The sense resistor and the sense device carry a second current proportional to the first current and generate a sensed potential across the sense resistor. The circuit further includes a variable reference current source for providing a varying reference current. A reference potential is generated based on the varying reference current and compared with the sensed potential. The varying reference current is varied according to a ratio of the voltage across the sense device to the voltage across the pass device. The current sensing circuit is capable of accurate current sensing when the pass device is operated either in the linear mode or in the saturation mode.
According to one aspect of the present invention, the pass device and the sense device are MOS transistors and the varying reference current is varied according to a ratio of the drain-to-source voltage of the sense device to the drain-to-source voltage of the pass device.
According to another aspect of the present invention, the variable reference current sources includes a first current source for providing a fixed reference current and a computation block for generating the varying reference current. The computation block generates the varying reference current as a function of the fixed reference current scaled by the ratio of the voltage across said sense device to the voltage across said pass device.
In one implementation, the variable reference current source includes a first transconductance amplifier and a second transconductance amplifier for generating a first current and a second current, respectively. The first current has a value indicative of the voltage across the sense device and the second current has a value indicative of the voltage across the pass device. Furthermore, the variable reference current source includes a translinear circuit for generating the varying reference current based on a ratio of the first current to the second current provided by the first and second transconductance amplifiers.
In accordance with the present invention, a transconductance amplifier circuit is provided for use with the current sensing circuit. The transconductance amplifier provides an output current indicative of the voltage difference at its input terminals. The transconductance amplifier includes pnp bipolar transistors for realizing bipolar level shifting functions. The bipolar level shifts establish a voltage across a resistor equaling to the voltage difference at the input terminals of the transconductance amplifier. The current flowing through the resistor is an output current indicative of the voltage difference at the input terminals. The transconductance amplifier operates under a short-circuit load condition to provide accurate current sensing.